The present invention relates to a gasket and more particularly to a cylinder head gasket adapted to be disposed between a cylinder block and a cylinder head of a liquid cooled internal combustion engine.
The cylinder head gasket of the type mentioned above which is formed with at least one hole for a combustion chamber and at least one hole for a liquid coolant passage is known. In the conventional gasket of this type, a thin metal grommet is mounted on the peripheral edge portion of the hole for the combustion chamber to seal the latter when the gasket is incorporated in between the cylinder head and the cylinder block. Since the grommet is made from a thin metal sheet, the grommet tends to be easily broken. In order to eliminate this disadvantage of the conventional gasket, it has been usual to make the grommet of a material such as steel or stainless steel which has a considerable mechanical strength. In this case, however, another problem arises that the grommet is abnormally overheated to an extent that the normal running of the engine becomes impossible. That is, since the gasket body of the gasket is made of asbestos whose thermal conductivity is low and the material such as steel or stainless steel used is also low in thermal conductivity, the heat radiation from the grommet becomes insufficient.
In order to clarify the above disadvantage and other disadvantages of the conventional gasket, one example of the latter will be described with reference to FIG. 1.
The typical example of the conventional cylinder head gasket has a structure as shown, in cross section. The gasket body 1 of the cylinder head gasket is of asbestos and is formed with a hole 2 for a combustion chamber and holes 3 for liquid coolant passages. On a peripheral edge of the hole 2 of the gasket body 1 a grommet 4 provided by bending a thin metal plate to form an outwardly U-shaped cross section is attached such that the peripheral portion of the hole 2 is pinched between the legs of the U-shape. The cylinder head gasket is disposed in between a cylinder block 5 and a cylinder head 6. The grommet 4 is exposed to the combustion chamber of the engine and the holes 3 for the coolant passages are connected to openings of coolant chambers (not shown) of the cylinder head and the cylinder block. In this gasket structure, there are three problems which are listed below.
(1) The grommet 4 is made from a thin metal sheet. Therefore, when the engine is ignited, a large pressure due to an explosion of fuel is exerted on an outer surface 4b of a bent portion 4a of the grommet 4. Due to a compressive component of force generated by the pressure, which is in the thickness direction (vertical direction in FIG. 1) of the cylinder head gasket and acts particularly on outer peripheral face 4b of the bent portion 4a, a cracking is created in the bent portion 4a, causing the grommet 4 to be broken. When the grommet 4 is broken, the sealing function thereof may become unreliable. PA1 (2) The breakdown of the grommet 4 is a very important problem for a high power internal combustion engine. In order to overcome this problem the grommet 4 must be formed of a material such as stainless steel or steel which has a high mechanical strength. This is important when the grommet 4 is used in a high power internal combustion engine. Such material, however, has a relatively low thermal conductivity. Furthermore, since the grommet 4 is prepared from a thin sheet of such material, the amount of heat transmission thereof is also relatively small. Of course, the thermal conductivity of asbestos is very low. Therefore, when the engine is ignited, the temperature of the grommet 4 becomes abnormally high. Due to this fact, there may be a possibility that the grommet 4 itself functions as an ignitor causing the operation of the internal combustion engine to be unstable. Further, there may be a possibility that the cylinder block and/or the cylinder head will melt due to the abnormally high temperature of the grommet 4, causing the engine to be broken. PA1 (3) The gasket must function to gas-seal the combustion chamber of the engine by means of the grommet 4 disposed in between the cylinder block 5 and the cylinder head 6 and must function to liquid-seal the coolant chamber of the engine. However, it is difficult to determine the dimensions of the gasket body and the grommet 4 of the gasket so as to satisfy both sealing requirements simultaneously.